Novel Layered Semiconductor Quantum Structures in New Material Systems --- III-Nitride Quantum Cascade Emitters and Detectors, Light Emission in Fe:ZnSe with Quantum Cascade Laser Pumping, Theoretical Methods with Real-time Green's Functions, and Hyperbolic Metamaterials

Abstract

This dissertation focuses on the theory and the experimental implementation of novel phenomena and devices related to thin layered semiconductor quantum structures, especially in new material systems.

We explore quantum cascade (QC) structures in III-nitride material systems with large conduction band offsets and high longitudinal optical (LO) phonon energy. The first mid-infrared (mid-IR) intersubband (ISB) emission in III-nitride QC structures is demonstrated. The emission is purely transverse magnetic polarized, which verifies its ISB origin. The center of emission at 5 &mum is clearly separated from the blackbody radiation, and agrees very well with the design. We also report the first QC detector in metal organic chemical vapor deposition grown III-nitride superlattices. A current responsivity of up to &sim 100 &muA/W and a specific detectivity of up to &sim 1 &times 10<super>8</super> Jones are recorded.

The non-equilibrium Green's function (NEGF) theory is employed for a profound understanding of the subband structure in layered semiconductors. A general model for handling the effects of the interface roughness in thin layered semiconductors is developed within the framework of NEGF. The model predicts an effective grading in the interfaces and an extraordinary fast roughness scattering lifetime (< 0.1 ps), both of which are re-affirmed by the experimentally observed energy shift and unusual broadening in the ISB transitions in III-nitride superlattices. These quantitative results are included in the design of the III-nitride QC emitters and detectors.

The ultra fast ISB scattering lifetime (sub-ps) is detrimental for ultra-short pulsed lasing, thus we explore other routes to this goal. We study light emission from a Fe<super>2+<super>:ZnSe polycrystal with QC laser pumping. Photoluminescence is achieved with its spectral and time-dependent characteristics examined. Effectively, energy stored in the upper state of the QC laser is transferred to the upper manifold of Fe<super>2+<super>:ZnSe with a 10<super>6<super> - 10<super>8<super> times longer lifetime.

Layering of semiconductor structures not only delivers ISB transitions but also enables strong optical anisotropy. And here the mid-IR semiconductor layered metamaterials (hyperbolic metamaterials) are also studied. The metamaterial characteristics with varied layer thicknesses are investigated. We also explore the possibility of inserting a mid-IR QC laser medium into the layered structure aiming at a reduction of the optical loss. The negative refraction characteristics are retained in the composite structure, and smooth carrier transport is achieved.